1. Field of the Invention
The present invention relates to a DC-DC converting apparatus, and particularly, to an improved DC-DC converting apparatus for obtaining a constant output voltage via an auxiliary circuit.
2. Description of the Related Art
A DC-DC converting apparatus is a voltage converter for effectively converting a DC input voltage into a constant DC output voltage, and examples of the converter include a bridge circuit, Bulk, Boost and other circuit elements. That is, a desired constant output voltage is obtained via a conversion from a large unsteady input DC or AC power source through the use of such voltage converting circuits.
In operation, an unsteady input DC power source has an input voltage value which tends to fluctuate, and the smaller that the fluctuation range is, the higher that the efficiency of the voltage converting circuit is (i.e., the lower the loss of the circuit is). Therefore, in order to obtain a highly efficient converting circuit, the gain range must be small.
However, if a voltage of an input power source has a large fluctuation range, a highly efficient converting circuit cannot be directly used because existing highly efficient converting circuits usually have a small gain range.
In order to utilize a highly efficient converting circuit which can be directly used with an input power source which has a large voltage fluctuation range, an auxiliary converting circuit is usually provided between the converting circuit and the power source. When an input voltage range of a converting circuit exceeds a predetermined range, an auxiliary converting circuit narrows the wide input voltage range so as to adapt for the converting circuit. For example, if a predetermined input voltage range of a converting circuit is 24V˜36V. If the input voltage is 12V˜48V, then it exceeds the predetermined input range. In this case, the auxiliary converting circuit typically reduces the width of the input voltage to 24V˜36V first and then applies it to an input terminal of the converting circuit.
For example, Chinese Patent CN200780051915.3 provides a DC/DC converter including a pre-adjuster stage including a Buck converter; and a post-converter stage including a charge pump. The duty cycle of the pre-adjuster stage is controlled by a feedback path extending from the output terminal of the pre-adjuster stage or the post-converter stage. Based on the duty cycle, the pre-adjuster lowers the input DC voltage at a variable amount, and the post-converter increases or reduces the voltage at the output of the pre-adjuster at a positive or negative integer or fraction number.
However, such a design produces other problems. For example, no matter whether the input voltage is in the predetermined range or exceeds the predetermined range, the auxiliary converting circuit coupled to the main circuit is always in an operating state and consumes energy since it precedes the converting circuit. That is, in a case involving no auxiliary converting circuit (for example, the input circuit is in the predetermined range), the auxiliary converting circuit provided between the main circuit and the power source inevitably consumes energy.
Thus, such a design does not enhance the integral efficiency of the circuit, because the preceding auxiliary converting circuit would result in a loss while narrowing the input voltage range, and thus the advantage (the reduced loss) resulted from a small gain range is balanced out.